Tuned fiber optic interleaver

Abstract

An optical fiber interleaver and method for aligning the output channels to a set of nominal values in frequency space. The interleaver is formed of two optical fibers joined together at fused input and output couplers to define an input section, an interferometer section and an output section. Within the interferometer section, an element is arranged to vary the optical path length of at least one of the fibers in response to a feedback signal. The feedback signal is generated by examination of the intensity of a monitor signal at the output section that was applied at the input section.

What is claimed is:  
     
       1. An optical interleaver for separating an input DWDM signal occupying a nominal frequency band into two output signals, one comprising a plurality of odd optical channels and the other comprising a plurality of even optical channels within said frequency band, said interleaver comprising, in combination: 
       a) a first optical fiber having opposed ends;  
       b) a second optical fiber having opposed ends;  
       c) said first and second fibers being fused together to form first and second couplers to thereby define an input section before said first coupler, an interferometer section between said first and second couplers and an output section after said second coupler;  
       d) a first tap coupler within said input section for receiving a monitor signal having a nominal frequency;  
       e) a second tap coupler within said output section for providing an interleaver output signal;  
       f) an optical path length adjustment element for shifting said monitor signal in frequency space, said optical path length adjustment element being responsive to a feedback signal;  
       g) a circuit for generating said feedback signal; and  
       h) said circuit being responsive to said output signal.  
     
     
       2. An interleaver as defined in  claim 1  wherein said optical path length adjustment element is arranged to affect at least one optical fiber within said interferometer section physically in response to said feedback signal. 
     
     
       3. An interleaver as defined in  claim 2  further characterized in that said feedback signal is periodic. 
     
     
       4. An interleaver as defined in  claim 2  wherein said optical path length adjustment element includes piezoelectric material. 
     
     
       5. An interleaver as defined in  claim 4  wherein said optical path length adjustment element comprises a piezoelectric annulus. 
     
     
       6. An interleaver as defined in  claim 4  wherein said optical path length adjustment element comprises s pair of piezoelectric elements arranged to apply a controllable compressive force to said at least one fiber. 
     
     
       7. An interleaver as defined in  claim 2  wherein said optical path length adjustment element is arranged to apply heat to said at least one fiber. 
     
     
       8. An interleaver as defined in  claim 2  wherein said optical path length adjustment element comprises a stepper motor. 
     
     
       9. An interleaver as defined in  claim 1  wherein said circuit includes a first photodetector. 
     
     
       10. An interleaver as defined in  claim 9  wherein said circuit additionally includes: 
       a) a bandpass filter; and  
       b) said filter is arranged to provide an output to said first photodetector.  
     
     
       11. An interleaver as defined in  claim 10  wherein said bandpass filter is centered at the nominal frequency of said monitor signal. 
     
     
       12. An interleaver as defined in  claim 11  wherein said monitor signal lies outside said frequency band. 
     
     
       13. An interleaver as defined in  claim 11  further including a processor for generating said feedback signal in response to an output from said first photodetector. 
     
     
       14. An interleaver as defined in  claim 13  further including a second photodetector arranged in parallel with said first photodetector. 
     
     
       15. A method for aligning optical channels at an output of an interleaver to a set of nominal values within a nominal frequency band in frequency space, said interleaver being of the type that includes a first and a second optical fiber, each fiber having opposed ends and fused together between their ends for form first and second couplers to define an input section before the first coupler, an interferometer section between the first and second couplers, and an output section after the second coupler, said method comprising the steps of: 
       a) inputting a monitor signal of nominal frequency outside said nominal frequency band at said input section; then  
       b) observing said monitor signal at said output section; then  
       c) adjusting the optical path length through at least one of said fibers within said interferometer section in response to said observed monitor signal; and then  
       d) repeating steps b and c as long as said center frequency of said monitor signal approaches said nominal monitor signal frequency.  
     
     
       16. A method as defined in  claim 15  wherein the step of observing said monitor signal additionally comprises the steps of: 
       a) applying said monitor signal to a passband filter having a center frequency that corresponds to said nominal monitor signal frequency; and then  
       b) detecting the intensity of the output of said passband filter.  
     
     
       17. A method as defined in  claim 16  wherein the step of adjusting additionally comprises the step of adjusting the optical path length through at least one of said fibers within said interferometer section. 
     
     
       18. A method as defined in  claim 17  wherein the step of adjusting the optical path length through at least one of said fibers within said interferometer section additionally comprises the steps of: 
       a) engaging said at least one fiber to a fiber adjustment element so that the optical path length of said fiber is responsive to said fiber adjustment element; and then  
       b) applying a feedback signal to energize said fiber adjustment element in response to said signal.  
     
     
       19. A method as defined in  claim 18  wherein said signal is a periodic signal. 
     
     
       20. A method as defined in  claim 18  wherein said fiber adjustment element includes piezoelectric material. 
     
     
       21. An interleaver as defined in  claim 20  wherein said fiber adjustment element comprises a piezoelectric annulus. 
     
     
       22. An interleaver as defined in  claim 20  wherein said fiber adjustment element comprises a pair of piezoelectric elements arranged to apply a controllable compressive force to said at least one fiber. 
     
     
       23. An interleaver as defined in  claim 18  wherein said fiber adjustment element is arranged to controllably apply heat to said at least one fiber. 
     
     
       24. An interleaver as defined in  claim 18  wherein said optical path length adjustment element comprises a stepper motor. 
     
     
       25. A method as defined in  claim 17  wherein the step of adjusting the optical path length of at least one of said fibers comprises the additional step of adjusting said optical path length until said intensity of said output of said passband filter is a maximum. 
     
     
       26. A method as defined in  claim 17  comprising the additional step of repeating steps b and c of  claim 11  until said intensity of said output of said passband filter no longer increases.